The present invention relates to a substituted triazolo-pyridazine derivative and to its use in therapy. More particularly, this invention is concerned with a particular substituted 1,2,4-triazolo[4,3-b]pyridazine derivative which is a GABAA receptor ligand and is therefore useful in the therapy of deleterious mental states.
Receptors for the major inhibitory neurotransinitter, gamma-aminobutyric acid (GABA), are divided into two main classes: (1) GABAA receptors, which are members of the ligand-gated ion channel superfamily; and (2) GABAB receptors, which may be members of the G-protein linked receptor superfamily. Since the first cDNAs encoding individual GABAA receptor subunits were cloned the number of known members of the mammalian family has grown to include at least six xcex1 subunits, four xcex2 subunits, three xcex3 subunits, one xcex4 subunit, one xcex5 subunit and two xcfx81 subunits.
Although knowledge of the diversity of the GABAA receptor gene family represents a huge step forward in our understanding of this ligand-gated ion channel, insight into the extent of subtype diversity is still at an early stage. It has been indicated that an xcex1 subunit, a xcex2 subunit and a xcex3 subunit constitute the minimum requirement for forming a fully functional GABAA receptor expressed by transiently transfecting cDNAs into cells. As indicated above, xcex4, xcex5 and xcfx81 subunits also exist, but are present only to a minor extent in GABAA receptor populations.
Studies of receptor size and visualisation by electron microscopy conclude that, like other members of the ligand-gated ion channel family, the native GABAA receptor exists in pentameric form. The selection of at least one xcex1, one xcex2 and one xcex3 subunit from a repertoire of seventeen allows for the possible existence of more than 10,000 pentameric subunit combinations. Moreover, this calculation overlooks the additional permutations that would be possible if the arrangement of subunits around the ion channel had no constraints (i.e. there could be 120 possible variants for a receptor composed of five different subunits).
Receptor subtype assemblies which do exist include, amongst many others, xcex11xcex22xcex32, xcex12xcex22/3xcex32, xcex13xcex2xcex32/3, xcex12xcex2xcex31, xcex15xcex23xcex32/3, xcex16xcex2xcex32, xcex16xcex2xcex4 and xcex14xcex2xcex4. Subtype assemblies containing an xcex11 subunit are present in most areas of the brain and are thought to account for over 40% of GABAA receptors in the rat. Subtype assemblies containing xcex12 and xcex13 subunits respectively are thought to account for about 25% and 17% of GABAA receptors in the rat. Subtype assemblies containing an xcex15 subunit are expressed predominantly in the hippocampus and cortex and are thought to represent about 4% of GABAA receptors in the rat.
A characteristic property of all known GABAA receptors is the presence of a number of modulatory sites, one of which is the benzodiazepine (BZ) binding site. The BZ binding site is the most explored of the GABAA receptor modulatory sites, and is the site through which annxiolytic drugs such as diazepam and temazepam exert their effect. Before the cloning of the GABAA receptor gene family, the benzodiazepine binding site was historically subdivided into two subtypes, BZ1 and BZ2, on the basis of radioligand binding studies. The BZ1 subtype has been shown to be pharmacologically equivalent to a GABAA receptor comprising the xcex11 subunit in combination with xcex2 subunit and xcex32. This is the most abundant GABAA receptor subtype, and is believed to represent almost half of all GABAA receptors in the brain.
Two other major populations are the xcex12xcex2xcex32 and xcex13xcex2xcex32/3 subtypes. Together these constitute approximately a further 35% of the total GABAA receptor repertoire. Pharmacologically this combination appears to be equivalent to the BZ2 subtype as defined previously by radioligand binding, although the BZ2 subtype may also include certain xcex15-containing subtype assemblies. The physiological role of these subtypes has hitherto been unclear because no sufficiently selective agonists or antagonists were known.
It is now believed that agents acting as BZ agonists at xcex11xcex2xcex32, xcex12xcex2xcex32 or xcex13xcex2xcex32 subunits will possess desirable anxiolytic properties. Compounds which are modulators of the benzodiazepine binding site of the GABAA receptor by acting as BZ agonists are referred to hereinafter as xe2x80x9cGABAA receptor agonistsxe2x80x9d. The xcex11-selective GABAA receptor agonists alpidem and zolpidem are clinically prescribed as hypnotic agents, suggesting that at least some of the sedation associated with known anxiolytic drugs which act at the BZ1 binding site is mediated through GABAA receptors containing the xcex11 subunit. Accordingly, it is considered that GABAA receptor agonists which interact more favourably with the xcex12 and/or xcex13 subunit than with xcex11 will be effective in the treatment of anxiety with a reduced propensity to cause sedation. Also, agents which are antagonists or inverse agonists at xcex11 might be employed to reverse sedation or hypnosis caused by xcex11 agonists.
The present invention is directed to a compound according to Formula (I) and pharmaceutically acceptable salts thereof that are GABA-A Alpha 2 and/or 3 ligands useful in the treatment of disorders of the central nervous system: 